1. Field of the Invention
The invention relates to a process for manufacturing a preform for a load path aligned fiber composite structure. The invention also relates to the use of such a process in the manufacture of load path aligned fiber composite structures. Finally, the invention relates to a device for carrying out the process.
2. Background Information
At the construction of vehicles of all kinds, particularly at the construction of aircrafts and spacecrafts, but also in other branches of industry such as mechanical engineering, there is an increasing need for strong and yet lightweight, cost-efficient materials. Especially fiber composite materials offer an outstanding lightweight construction potential. The principle resides in the fact that particularly high-strength and stiff fibers are embedded in a matrix in a load path aligned fashion, thus producing components having outstanding mechanical properties by using previous techniques and having a weight which at a comparable performance is typically 25% less than that of aluminum structures and 50% less than steel structures. A drawback is the high material costs and particularly the laborious and mainly manual fabrication.
Accordingly, there is a desire for an automated manufacture facilitating machine positioning of the fibers in space. Nowadays, fiber-reinforced plastic materials are characterized by an extremely high strength and stiffness at a low weight, particularly if oriented long fibers, for instance carbon fibers, are used. They also have a high weight-specific energy absorption potential and good fatigue characteristics.
Up to now this is achieved by endless fibers being incorporated in a matrix (e.g. epoxy resin) in a load path aligned fashion. Depending on the direction of reinforcement, anisotropic materials having direction-dependent mechanical properties can be produced. For instance, a material can have characteristics which are different from each other in the length and in the width of the material. Already today, a high percentage of the structural weight in modern aircrafts and spacecrafts, is made up of fiber-reinforced plastic materials.
Currently, the most important manufacturing process is based upon the so-called prepreg technology. This technology involves positioning the reinforcing fibers in a parallel (unidirectional) fashion and embedding the fibers in a matrix. After a curing step, semi-finished products are produced which are rolled up as a thin layer. During processing, these layers are cut corresponding to the contour of the component and are laminated in a tool layer by layer and preferably by hand. Thereafter, curing takes place under pressure and temperature inside an autoclave. The resulting components exhibit a very high light construction potential, but the manufacture is laborious and expensive. For this reason material searchers have for long dealt with the question in which way fibers can be positioned aligned to the load path and three-dimensionally and with a contour which matches the final contour of the component as closely as possible, in an automated process.
To produce fiber composite structures with load path aligned fibers, so-called preforms as textile semi-products have been manufactured up to present for selected applications in addition to prepregs. These are mostly two- or three-dimensional structures having a load path aligned fiber orientation. Up to present endless fibers are placed in the load direction and prefixed by using means and techniques from textile engineering, normally sewing, knitting or the like. Examples of devices and processes for producing such preforms are disclosed in DE 30 03 666 A1, DE 196 24 912, DE 197 26 831 A1 and DE 100 05 202 A1.
From U.S. Pat. No. 4,952,366 A1 a comparable process for manufacturing a preform and a composite structure from this preform is known, wherein a pregreg is reinforced with an endless fiber in order to produce the preform. During the transport of the prepreg the endless fiber is initially fixed to the pregreg through vacuum suction. Hence, a transport of fiber layups on a conveyor belt in such a way that these fiber layups are drawn onto the conveyor belt by means of vacuum, is known from this prior art.
From DE 30 48 367 C1 a process and a device for spreading a fiber strand are known. In this process the fiber strand is penetrated and fanned out as widely as possible by a convex surface.
From CH 450239 there are also known a process and a device for spreading a fiber strand. In this process the fiber strand is passed over fixed strips in a meandering fashion. A guide surface of the strip is convexly bent in arc-shaped fashion.
From US 2002/0123819 A1 a system for automatic control of the spreading of a textile layer is known. The textile layer is composed of a plurality of fiber strands. This system includes a measuring device for measuring the position of the longitudinal edges of each fiber strand, a width adjustment device for the individual adjustment of the width of each fiber strand, a position adjustment device for the adjustment of the position of each rope, and a control device for controlling the spreading operation in such a manner that the textile layer has a predetermined width and position. During this operation the fiber strands are pulled over bent rods.
From DE 695 00 513 T2 a segmented fiber laying head for laying and pressing fibers strands against a preform is known. Due to its segmentation the fiber laying head is elastic, so that the pressing force can be better adjusted for irregular moulds. The laying head includes a pressure roller by which individual long fiber strands can be pressed against the preform. For forming the segmented fiber laying head this pressure roller is composed of many individual discs. Totally sixteen individual fiber strands are passed via fiber strand guides to the fiber laying head.
From EP 0 491 353 A1 a multiple axes fiber lay-up machine comprising a laying head on a robot arm is known. This laying head includes a pressure roller by means of which fiber strands can be compacted and pressed-on. The laying head processes a roving into a band and places the band onto a preform supported for rotation. The rovings are previously impregnated with resin. The individual elements are temperature-controlled, in order to adjust the adhesiveness of the resin for the respective processing step. In particular, the compacting roller can be heated for increasing the adhesiveness for the pressing-on operation. In this case, too the rovings are directly passed from a supply reel via fiber guides to the laying head.
From FR 2882681 there is also known a fiber lay-up machine for producing fiber-reinforced composite structures. The same includes a laying head for applying fiber strands. The laying head also includes a lay-up roller for the placement of the fibers onto a preform. Further the laying head, which is articulated to the end of a robot arm, is provided with means for applying resin to each fiber shortly before laying. Accordingly, a thermoplastic material as a binder can be applied by the head. The head also includes a cutting device for cutting the fibers strands to the desired lengths. This cutting device is arranged upstream of the device for applying resin. A transfer of the fiber strands from the cutting device to the lay-up roller takes place by means of air. In addition, a cooling device for cooling the fiber strands contacting the lay-up roller is provided for adjusting the adhesiveness. In the device for applying resin, heating elements can be provided to adjust the viscosity of the resin for the purpose of dosing and applying. The laying head is supplied with a plurality of fiber strands which are placed side by side and combined into a fiber band or fiber tape. For this purpose a transfer device is provided guiding the endless fiber strands from the supply reel up to the laying head.
From DE 103 01 646 A1 a filament or fiber laying as well as a process for its manufacture are known. In this process individual strand pieces of fiber strand material are laid without tension. This is performed by a plurality of combined laying and separation devices which in addition to the laying function for laying the individual fiber strand also have a separating function for separating the fiber strand piece. The laying and separation device is tubular and can have a certain spreading function by which a fiber strand which originally has a round cross section is changed into a flat and more rectangular cross section. The laying normally takes place with parallel adjacent strand pieces. The fixing of the laid individual fiber strand pieces to the subsurface can be performed by clamping means having an additional spreading effect or by means of gluing.
However, the known processes for manufacturing preforms are complicated concerning their implementation and process technique. Particularly for components where curved load path lines with a varying density are to be expected, it is not possible with previous processes to manufacture a correspondingly load path aligned component. Particularly, the fibers cannot be oriented arbitrarily along defined curved paths and the fiber content cannot be locally varied.